1. Field of the Invention
The present invention relates to, for example, a position information estimation system that estimates position information of a capsule endoscope inserted into a subject.
2. Description of the Related Art
Conventionally, endoscopes have been in widespread use as medical observation apparatuses that are inserted into subjects to enable in-vivo observation. In recent years, endoscopes (capsule endoscopes) to be swallowed have been developed also, each equipped with devices, such as an imaging device and a communication device that wirelessly transmits image data captured by this imaging device, inside a capsule-shaped casing. A capsule endoscope has the function of, after being swallowed through the mouth of a subject for in-vivo observation, sequentially imaging internal organs such as an esophagus, a stomach, and a small intestine while moving inside these organs along with the peristaltic movement of these organs, until the natural egestion of the endoscope from the subject.
While the capsule endoscope moves in a body cavity, image data captured by the capsule endoscope in the body cavity are sequentially transmitted to the outside of the body by radio communication. Then, these image data are accumulated in a memory provided inside or outside of a receiving device outside of the body, or are displayed as images on a display provided to the receiving device. A doctor or a nurse is enabled to make diagnoses based on images that have been obtained by importing the image data, accumulated in the memory, to an information processor through a cradle having the receiving device inserted therein, and are displayed on a display of this information processor, or alternatively, images that have been received by the receiving device and are displayed on the display thereof.
Incidentally, for receiving a radio signal from a capsule endoscope, a receiving device in general has two or more antennas placed outside of a subject in a dispersed manner, and is configured to select one of the antennas that shows the highest signal level in the reception, and receive the radio signal via the thus selected antenna. One known example of thus configured receiving devices is a receiving device that switches reception between two or more receiving antennas placed outside of a subject and estimates the position of a capsule endoscope, which is the source of a radio signal, based on the strengths of electric field received by the respective receiving antennas.
Examples of a position information system that estimates the position of the above described capsule endoscope includes a moving state detection system disclosed by Japanese Unexamined Patent Publication No. 2006-26163. The moving state detection system detects a moving state of a capsule endoscope that moves inside a subject and outputs a sensor signal that attenuates in accordance with the propagation distance thereof. This moving state detection system includes a moving state derivation unit that, after a receiving antenna has received a sensor signal, derives the moving state based on the signal level in the reception.
Further, this moving state detection system includes a determination unit that determines the moving state of the capsule endoscope based on the selection as to which one of receiving antennas to use for receiving a sensor signal. The determination unit determines whether the capsule endoscope is currently in a low-speed moving state or in a high-speed moving state. This determination unit enables efficient diagnoses that only require, in checking images captured by the capsule endoscope, checking of some images in a group of images that corresponds to the capsule endoscope determined to be in the low-speed moving state.